1. Field of the Invention
The present invention relates to an optical characteristic measuring apparatus for measuring a spectral characteristic of a sample exhibiting fluorescence.
2. Description of the Related Art
In recent years, many of white paper and fabrics are treated by a fluorescent whitening agent (FWA). It is impossible or difficult to precisely evaluate the whiteness (degree of whiteness or brightness) or the hue of these products, without considering an influence of fluoresced light. In view of the above, there is a demand for improvement in colorimetry of FWA-treated paper or fabrics, considering an influence of fluoresced light.
Generally, a visible optical characteristic of a reflecting sample is expressed by a relative ratio to the perfect white. Specifically, a visible optical characteristic of a reflecting sample is expressed by the total spectral radiance factor B(λ). The total spectral radiance factor B(λ) is the ratio of light emitted from a reflecting sample in a certain illuminating condition and a certain receiving condition, to light emitted from a perfect reflecting diffuser in the identical illuminating and receiving conditions at each wavelength λ.
In an FWA-treated sample i.e. a sample containing a fluorescent material (hereinafter, called as a fluorescent sample), a color of fluoresced light is observed as an objective color, wherein reflected light is superimposed on the fluoresced light. In other words, light emitted from a fluorescent sample is given as the sum of reflected light (reflected light component) and fluoresced light (fluorescent light component) from a fluorescent sample. Accordingly, similarly to the above, the total spectral radiance factor B(λ) of a fluorescent sample is given as the sum of the reflection spectral radiance factor R(λ) and the fluorescent spectral radiance factor F(λ), which are the ratios of light reflected and fluoresced from the fluorescent sample in a certain illuminating condition and a certain receiving condition respectively, to light from a perfect reflecting diffuser in the identical illuminating and receiving conditions.
The above-mentioned perfect reflecting diffuser has no fluorescence, and the reflectivity thereof has no dependence on the wavelength of illumination light. Accordingly, the above-mentioned total spectral radiance factor B(λ), reflection spectral radiance factor R(λ) and fluorescent spectral radiance factor F(λ) are expressed as intensity ratios of the light emitted, reflected and fluoresced from the sample respectively, to the illumination light at each wavelength, with a suitable proportional constant. An object of the colorimetry is to obtain a measurement value analogous to visual observation. The color of a fluorescent sample is observed as an objective color, and accordingly is expressed by the total spectral radiance factor B(λ), from which the colorimetric values are derived.
The CIE (International Committee of Illumination) defines spectral distributions (spectral intensities) of several standard illuminations such as Illuminants D65 and D50, (daylight), Illuminant A (incandescent lamp), and Illuminants F1 through F12 (fluorescent lamp), as illumination light to be used in colorimetry. For the evaluation of fluorescent samples, Illuminant D65 is generally used. The spectral excitation-fluorescence characteristics of a fluorescent sample is expressed by the Bi-spectral Luminescent Radiance Factor F(μ,λ), which is the matrix data showing the intensity of fluoresced light at wavelength λ excited by monochromatic light at wavelength μ for illuminating the fluorescent sample surface with a unit intensity.
For instance, JP 2006-292510A (D1) discloses a practical method for obtaining the total spectral radiance factor B(λ). D1 discloses a method comprising: calculating a total spectral radiance factor of a sample illuminated by test illumination, based on a bi-spectral fluorescent radiance factor or a bi-spectral radiance factor analogous to the sample, a spectral distribution of the test illumination, spectral distributions of first actual illumination light and second actual illumination light whose relative spectral distributions are different between an excitation wavelength region and a fluorescent wavelength region, and an actually measured spectral distribution of light emitted from the sample illuminated by the first actual illumination light and the second actual illumination light. The test illumination is illumination light to be used in evaluating an optical characteristic such as standard illuminant D65. The spectral distribution of emitted light is measured by e.g. a polychromator (spectral analyzer). More specifically, light emitted from a sample is dispersed by a diffraction grating provided in a polychromator at each wavelength, and the wavelength components of the light are incident into light receiving elements of a sensor array, respectively. An electric charge of each of the wavelength components incident into the respective corresponding light receiving elements is accumulated in accordance with the received light amount, and the accumulated electric charges are converted into electric signals for outputting. The sensor array is an image sensor such as a CCD (Charge-Coupled Device) sensor, an MOS (Metal Oxide Semiconductor) sensor, or a CMOS (Complementary Metal Oxide Semiconductor) sensor.
In an optical characteristic measuring apparatus using the method recited in D1, there is no need of using a fluorescent standard sample, and performing a cumbersome calibration using the fluorescent standard sample, thereby simplifying the measuring method or enhancing the measurement efficiency.
However, in the method recited in D1, optical characteristics of plural samples containing a fluorescent material cannot be accurately measured in a short time.